Which animals have more than two biological sexes and how are they defined?
Executive summary
Some animals and many non-animal eukaryotes defy the simple “two sexes” story: examples range from classic hermaphrodites (snails, earthworms) to species with genetic or behavioral systems that scientists have argued amount to three or even four reproductive roles, and single-celled eukaryotes with dozens of mating types; how these are counted depends on the definition of “sex” being used (gamete type, mating partners, parental number) [1] [2] [3]. Debates in the literature show that labeling a system “more than two sexes” is both biologically meaningful in some cases and contentious in others — scholars warn that different criteria yield different answers [1] [4].
1. How biologists define “sex,” and why that matters
Biologists commonly define sex by gamete size (anisogamy: eggs vs sperm), by who an individual can mate with (mating types), and by parental number (how many distinct parental roles are required), and these three axes do not always point to the same counting scheme — so whether a system has “more than two sexes” depends on which axis is emphasized [1]. The gamete-size definition makes most animals dichotomous because sexual reproduction typically involves large and small gametes, but the mating-type concept used for fungi and protists lets systems carry many discrete, incompatible mating classes that function like additional sexes for partner choice [1] [5].
2. Hermaphrodites and sequential sex changers: both sexes, not extra sexes
Many animals are hermaphroditic (simultaneous or sequential), meaning individuals have both male and female reproductive function or can change sex during life — classic examples include many snails and earthworms — but these are not extra sexes under the gamete-size metric because only two gamete classes exist (egg and sperm), even if one individual can produce both [2] [6]. These strategies increase mating flexibility and population-level reproductive options without requiring a third gamete type, which is why most researchers treat hermaphroditism as sex variation rather than adding a new sex category [1].
3. Social insects and caste systems: more castes, not necessarily more sexes
Colonies of ants, bees and wasps have queens, male drones, and worker castes, and some popular accounts present these as three “sexes,” but specialists caution that worker castes are typically sterile or non-reproductive females and therefore do not constitute a separate gamete-producing sex under standard definitions [7] [1]. That said, unusual genetic systems within some harvester ants (Pogonomyrmex) require queens to mate with two genetically distinct male types to produce different offspring castes, prompting honest debate among evolutionary biologists about whether such systems represent emergent “more-than-two-sex” dynamics or complex two-sex interactions with caste-determining genetics [1].
4. Birds with “four sexes”: the white-throated sparrow case
The white-throated sparrow has been described as having “four sexes” because a chromosomal inversion creates two behavioral–morph classes in each gamete-sex, producing four mating classes (two morphs of males and two of females) with strong assortative pairing driven by behavior and genetics; some authors and commentators call this a four-sex system while others insist it’s better framed as a two-gamete system overlaid by behavioral polymorphism — the disagreement highlights the definitional choice at play [4].
5. Protists and fungi: many mating types that act like sexes
Single-celled eukaryotes provide unequivocal examples where more-than-two reproductive classes exist: the ciliate Tetrahymena thermophila has seven mating types, and some fungi possess tens to thousands of mating types, which function as mutually incompatible partner classes and therefore behave like multiple sexes with respect to who can mate with whom [3] [5] [7]. These systems show that when “sex” is operationalized as mutually compatible mating classes rather than gamete size, many more than two functional sexes occur across life [5] [3].
6. Bottom line and the ongoing controversy
The short, evidence-based conclusion is: animals do not generally evolve extra gamete classes (most animals remain bounded by eggs and sperm), but biological realities that expand reproductive roles exist — hermaphrodites and sex changers, caste-dependent reproductive systems, bird morphs like the white-throated sparrow, and the harvester-ant hybrid system provoke debate — while protists and fungi routinely exhibit more-than-two mating types that act like sexes under the mating-type definition [1] [4] [3]. Which taxa “have more than two sexes” therefore depends on the definitional lens chosen, and reputable sources explicitly debate where to draw the line [1].